Over 15% of the total energy consumption is used for refrigeration. Now, the commonly used gas compression refrigeration technology has many disadvantages such as high energy consumption and environmental pollution, etc. Therefore, exploration of pollution-free and environment friendly refrigeration materials and development of novel refrigeration technologies with low energy consumption and high efficiency become very urgent in the whole world.
Magnetic refrigeration technology, as characterized by environment friendly, energy efficient, stable and reliable, has drawn great attention worldwide in recent years. Several types of giant magnetocaloric materials at room temperature and even high temperature zone were found successionally in US, China, Holland and Japan, which significantly increased the expectation for environment friendly magnetic refrigeration technology, e.g. Gd—Si—Ge, LaCaMnO3, Ni—Mn—Ga, La (Fe, Si)13-based compound, Mn—Fe—P—As, MnAs-based compound, etc. Common features of these novel giant magnetocaloric materials lie in that their magnetic entropy changes are all higher than that of the traditional magnetic refrigeration material Gd working around room temperature (R. T.), their phase-transition properties are of the first-order, most of them show strong magnetocrystalline coupling characteristics, and magnetic phase transition is accompanied with distinct crystalline structural transition. These novel materials also show different features. For example, Gd—Si—Ge is not only expensive but also requires further purification of the raw material while being prepared. And the raw materials used to prepare Mn—Fe—P—As and MnAs-based compound, etc. are toxic; NiMn-based Heusler alloy shows large hysteresis loss, and so on.
Among the several novel materials found in the past over ten years, La(Fe, Si)13-based compound is commonly accepted worldwide and has the highest potential for magnetic refrigeration application in a high temperature zone or even at R.T. This alloy has many characters shown as follows: the cost of its raw material is low; phase-transition temperature, phase-transition nature and hysteresis loss may vary upon component adjustment; its magnetic entropy change around R.T. is higher than that of Gd by one fold. In the laboratories/companies of many countries, La(Fe,Si)13-based magnetic refrigeration material has been used for prototype test, which proved its refrigerating capacity is better than that of Gd.
The phase-transition nature of La(Fe, Si)13-based compound varies with the adjustment of its components. For example, for the compound with low Si amount, its phase-transition property is normally of the first-order. Upon the increasing of Co content, the Curie temperature increases, the first-order phase-transition nature is weakened and gradually transited to the second order (no hysteresis loss for the second-order phase transition); thus hysteresis loss is decreased gradually. However, due to the component change and exchange interaction, the magnitude of magnetocaloric effect is also reduced in turn. Addition of Mn can lower the Curie temperature by impacting the exchange interaction; the first-order phase-transition nature is weakened; hysteresis loss is decreased gradually; and the magnitude of magnetocaloric effect is also reduced in turn. In contrast, it was found that replacement of La with small rare earth magnetic atoms (e.g. Ce, Pr, Nd) can enhance the first-order phase-transition nature; and increase hysteresis loss and the magnitude of magnetocaloric effect. It was expected that the first-order phase-transition La(Fe,Si)13-based compound showing a giant magnetocaloric effect can be used in magnetic refrigeration application in practice, so as to achieve ideal refrigerating effect.
However, the La(Fe,Si)13-based compound having a first-order phase-transition nature although shows giant magnetocaloric effect, is often accompanied with significant hysteresis loss, which results in heat leaking in the refrigeration cycles of a magnetic refrigerator. Refrigerating efficiency will be enormously reduced by the significant hysteresis loss resulted in the first-order phase-transition process.